Several gear failure cases and lessons learned in the development phase of aircraft high lift actuation
systems are presented, including leading edge geared rotary actuators, and trailing edge geared rotary
actuators, sector gears and pinions, and offset gearboxes. The high lift system of an aircraft, which
contains trailing edge flaps and/or leading edge slats, increases lift for takeoff, controls flight during
cruise, and reduces speed while increasing lift for shorter landing distance.
Many of these components contain highly loaded gears to increase the power to weight ratio. Because of
requirements on weight or envelope and consideration of cost, the gears are always designed to the limit
with reasonable margins of safety in a high lift system. The structure which supports the gears is limited in
size and simplified, and the gear material and heat treatment are selected for easy manufacturing.
Therefore, when misalignment and/or deflection of the gears are large enough to cause reduction in tooth
contact area, the stress on gears becomes large enough to cause damage. The failure modes can be
classified as spalling or pitting at the location of concentrated loads. Most of the problems can be
resolved by providing correct lead modification to alleviate the concentrated loading, while some need
increase of the gear diameters, design modifications, or introduction of materials with higher allowable.